Monohybrid Crosses - Online Tutor, Practice Problems & Exam Prep

A monohybrid cross involves the fertilization between two heterozygous organisms for a single gene, resulting in a characteristic genotypic ratio of 1:2:1 (1 homozygous dominant, 2 heterozygous, 1 homozygous recessive) and a phenotypic ratio of 3:1 (3 dominant phenotypes to 1 recessive phenotype). The Punnett square visually represents these outcomes, illustrating the inheritance patterns of alleles. Understanding these ratios is crucial for predicting genetic variations in offspring and forms the foundation for more complex genetic studies.

Monohybrid Crosses

In this video, we're going to begin our lesson on monohybrid crosses. A monohybrid cross is really just a cross-fertilization event between two monohybrid organisms. Recall from our previous lesson videos that monohybrids are organisms that are heterozygous for just one specific gene. That's because the root mono means "one".

If we take a look at our example image down below, we're showing you the Punnett Square of a monohybrid cross. A monohybrid cross involves two monohybrid organisms. At the top, we have a monohybrid organism, a parent that is heterozygous for one gene, the color gene if you will, where it has one dominant allele (one capital Y) and one recessive allele (one lowercase y). Its gametes are across the top here. Parent number two is exactly the same, another monohybrid, and its gametes are across the side.

When you fill out the Punnett square for this monohybrid cross, what you end up getting are a homozygous dominant possibility in the top left, a heterozygous possibility in the top right, another heterozygous possibility in the bottom left, and a homozygous recessive possibility in the bottom right. This is a monohybrid cross, a cross-fertilization between two monohybrid organisms, or two organisms that are heterozygous for one gene.

This concludes our brief introduction to monohybrid crosses. In our next video, we'll get to talk a little bit more about them. So, I'll see you all there.

Genotypic vs. Phenotypic Ratio

In this video, we're going to distinguish between the genotypic and phenotypic ratios. And so the genotypic ratio, as its name implies, is going to be the ratio of different genotypes observed amongst the offspring. And so the characteristic genotypic ratio from a monohybrid cross is 121, where there is 1 homozygous dominant individual for every 2 heterozygous individuals for every one homozygous recessive individual. Now the phenotypic ratio, as its name implies, is going to be the ratio of different phenotypes observed amongst the offspring. And so the characteristic phenotypic ratio from a monohybrid cross is 31, where there are 3 individuals that display the dominant phenotype and one individual that displays the recessive phenotype.

And so, of course, previously we had done a monohybrid cross here, and, of course, the results of the monohybrid cross are in this Punnett square. And what you can see is that, once again, the genotypic ratio shows the ratios of the genotypes. So there is again 1 homozygous dominant individual, 2 heterozygous individuals or possibilities, and 1 homozygous recessive possibility. And so that 121 is coming straight from that. The genotypic ratio is different from the phenotypic ratio, which looks at the phenotypes, and the phenotypes are the colors of the peas. And so amongst the offspring, there are 3 possibilities for the offspring to be yellow and one possibility for the offspring to be green. And so, the ratio is 31.

And those are the characteristic genotypic and phenotypic ratios of monohybrid crosses, and that's important to keep in mind. So this here concludes our introduction to genotypic and phenotypic ratios, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Monohybrid Crosses Example 1

So here we have an example problem that says capital T is the dominant allele for plant height resulting in tall plants, whereas lowercase t is the recessive allele for plant height resulting in short plants. A monohybrid cross of two heterozygous parents or one capital T one lowercase t, results in offspring having a tall plant to short plant ratio of which one of these four potential answer options down below. And so to solve this example problem, of course, we're going to need to perform the monohybrid cross that they're asking us about. And so of course, in order to perform a monohybrid cross, we first need to create our Punnett square, which is right here, and then we're going to be crossing two heterozygous parents. And so that means that we're going to have a capital T and a lowercase t for each of the gametes of the parents. And so when we fill this Punnett square out, what we get is two capital T's here, a capital T and a lowercase t here, another capital T and a lowercase t here, and two lowercase t's here. And so, again, this problem is asking us about the tall plant to short plant ratio. And, of course, tall plants and short plants are the phenotype, not the genotype. So it's asking us about the phenotypic ratio. And so here what we should write is phenotypic, because these are the phenotypes and we're looking for the phenotypic ratio. And, of course, it's tall plant to short plant. So we want the first number to be the tall plant and the second number to represent short plants. And so when we take a look at the individuals and the individual possibilities in this Punnett square, notice that three of these possibilities would result in tall plants because they have at least one dominant allele, at least one T. And so that means that there will be 3 tall plants, and this is going to be 3 tall plants for every one short plant. And so, really, this is the ratio that we are looking for, 3 tall plants for every one short plant, a 3 to 1 ratio. And the answer option that matches 3 to 1 is, of course, going to be answer option b, 3 to 1. So we can go ahead and indicate that b here is the correct answer and all of these other ones are incorrect. And so that concludes this example problem, and we'll be able to get some more practice applying these as we move forward. So I'll see you all in our next video.

Which of the following is the genotypic ratio of offspring created from a cross of two heterozygous parents for a single gene?

a) 2:2

b) 3:1 

c) 1:2:1 

d) 1:3

• A. 2:2
• B. 3:1
• C. 1:2:1
• D. 1:3

In a single gene cross between a homozygous dominant parent and a homozygous recessive parent, which generation is always completely heterozygous?

a) F1 generation.

b) F2 generation.

c) F3 generation.

d) P generation.

• A. F1 generation.
• B. F2 generation.
• C. F3 generation.
• D. P generation.

Which of the following single gene crosses would always result in all offspring with the dominant phenotype?

a) tt x tt. 

b) Tt x Tt. 

c) TT x tt. 

d) Tt x tt.

• A. tt x tt.
• B. Tt x Tt.
• C. TT x tt.
• D. Tt x tt.

Here’s what students ask on this topic:

What is a monohybrid cross?

A monohybrid cross is a genetic cross between two organisms that are heterozygous for a single gene. This type of cross focuses on the inheritance of one specific trait. The organisms involved have one dominant allele and one recessive allele for the gene in question. The Punnett square is often used to visualize the possible genetic outcomes of a monohybrid cross, which typically results in a genotypic ratio of 1:2:1 (1 homozygous dominant, 2 heterozygous, 1 homozygous recessive) and a phenotypic ratio of 3:1 (3 dominant phenotypes to 1 recessive phenotype).

What are the genotypic and phenotypic ratios in a monohybrid cross?

In a monohybrid cross, the genotypic ratio refers to the ratio of different genotypes among the offspring. The characteristic genotypic ratio is 1:2:1, meaning there is 1 homozygous dominant individual, 2 heterozygous individuals, and 1 homozygous recessive individual. The phenotypic ratio, on the other hand, refers to the ratio of different observable traits (phenotypes) among the offspring. The characteristic phenotypic ratio is 3:1, where 3 individuals display the dominant phenotype and 1 individual displays the recessive phenotype.

How do you use a Punnett square for a monohybrid cross?

To use a Punnett square for a monohybrid cross, follow these steps: 1) Write the alleles of one parent across the top and the alleles of the other parent down the side. 2) Fill in the squares by combining the alleles from the top and side. 3) Analyze the resulting genotypes to determine the genotypic ratio (1:2:1) and phenotypic ratio (3:1). This tool helps visualize the possible genetic outcomes and predict the traits of the offspring.

What is the significance of the 3:1 phenotypic ratio in a monohybrid cross?

The 3:1 phenotypic ratio in a monohybrid cross is significant because it reflects the principle of complete dominance. In this ratio, 3 out of 4 offspring display the dominant phenotype, while 1 out of 4 displays the recessive phenotype. This ratio helps geneticists understand how traits are inherited and predict the likelihood of certain traits appearing in the offspring. It also provides insight into the underlying genetic mechanisms, such as the presence of dominant and recessive alleles.

What is the difference between homozygous and heterozygous genotypes in a monohybrid cross?

In a monohybrid cross, a homozygous genotype consists of two identical alleles for a specific gene, either both dominant (AA) or both recessive (aa). A heterozygous genotype, on the other hand, consists of one dominant allele and one recessive allele (Aa). Homozygous individuals will express either the dominant or recessive phenotype, depending on their alleles, while heterozygous individuals will express the dominant phenotype due to the presence of the dominant allele.